Magnet control



Nov. 28, 1939.

R. O. WERTZ MAGNET CONTROL Original Filed March 9, 1935 l l i I I I I Il l I 1 I l I 1 I I 3nventor Q0y O. MfRTZ (Ittorueg Patented Nov. 28,1939 MAGNET CONTROL Roy 0. Wertz, to The Ohio Cleveland Heights, Ohio,assignor Electric Mfg. 00., Cleveland, Ohio,

a corporation of Ohio Application March 9, 1935, Serial No. 10,243

Renewed October 15, 1938 10 Claims.

This invention relates to control means for electromagnets and moreparticularly relates to means for automatically eliminating the residualmagnetism in electro-magnets.

One of the most common uses for electromagnets is in and about steelmills and foundries where they are usually operated in connection withtraveling or "Gantry cranes and the like. The crane operator controlsthe magnet operation as well as the movement of the crane about theplant. Obviously in controlling the many movements and functions of thecrane there must be provided numerous controls. Any simplification ofthe controls that can be made without losing their efiectiveness ishighly desirable from the standpoint of the operator.

In the operation of electric lifting magnets direct current of variouspotentials is usually supplied for energizing the same. A magneticallyoperated main circuit breaker, controlled by manual master switch at aremote point, makes and breaks the circuit to the magnet.

It is also important to have the circuit and switches so arranged that areverse current at a lower potential may be supplied to the magnet toreverse the polarity thereof for the quick elimination of the residualmagnetism that ordinarily remains in the magnet when the main current isdiscontinued. Failure to dispose of this residual magnetism prevents theload from being dropped quickly and in some instances permits portionsof the load to adhere to the magnet. If the magnet should be moved awayfrom the point where the load was dropped with some part of the loadadhering thereto, the adhering piece might finally fall and causedamage.

In the conventional installation, the master switch operates the maincontactor in the circuit breaker to connect and disconnect the magnetfrom the current source and to reverse the current to demagnetize themagnet, being provided with three positions, namely on, oii, andreversed. The operator moves the switch to "on a and the magnet isenergized. When he desires to release the material, he moves it toreverse and the current, usually at reduced potential, is directedthrough the windings in the opposite direction to demagnetize themagnet, permitting the load to be quickly released. The switch must beheld in the reverse position until the load is released, and then it maybe returned to o Should the control be held in reverse too long thepolarity would again be built up in the oppoa site direction to the mainpolarity; therefore it is necessary that the magnet should not be heldin the reverse position too long.

The present invention contemplates automatically applying the reversin gcurrent to the magnet ior a sufficient period of time to eliminate theresidual magnetism and then to discontinue the application of thereversing current. By providing for automatic application of thereversing current, several important advantages are obtained. The loadis discharged quickly when the control is moved to an off position. Theoperator does not need to judge or guess that surficient reverse currenthas to be applied. There is a positive assurance that the exact amountof reverse current to eliminate the residual magnetism will be applied.The Work of the operator is simplified in that he has less controls tomanipulate;

he need not see the load to determine if it is dropped and becausethecontrols are simplified he can devote more of his time to theremaining controls speeding up the other operations.

Referring to the drawing,

I have shown a pair of main conductors B connected to a source of directcurrent A, the polarity being indicated by the usual signs. Theconductors are connected to contacts C of a main circuit breaker orcontactor through blow out coils J adjacent the contactor. The circuitto the magnet is adapted to be completed through the blades D whichblades are operated in unison by a current applied to an operating coilE. The current to the operating coil is controlled by a manuallyoperable master switch H having a lead F connected to the main positiveconductor which,

when thrown to the on position, completes the circuit to the negativeconductor through the conductor G, and

operating coil E, the end of which is connected to the other main linenear to contact C. When the master switch H is thrown to the offposition this circuit is broken releases the armatures D, cuit.

and the operating coil opening the main cir- It is now necessary toeliminate the residual magnetism in the reversal, but which in maccomplished as follows.

magnet M which, as previously stated, was heretofore accomplished by amanual y improved circuit is The winding of the magnet M hasconsiderable inductance. Breaking the circuit causes a large voltagejump due to the self-inductance which may cause the potential across theterminals of the magnet to increase from 220 volts to 1000 volts orhigher. The voltage jump of course varies with the main appliedpotential and the flux of the magnet. This voltage jump or surge isutilized to put into operation a, relay K, which relay controls thereverse current.

The relay K is provided with a core having three windings L, N and O; Land N are so wound on the core as to be of the same polarity, and O iswound in the opposite direction to provide opposite polarity. Winding L,in series with a valve P, is connected across the terminals of themagnet. The valve P is preferably a material having the property ofdecreasing its ohmic resistance when the potential across it increases,one such material I have found particularly suitable is known to theindustry under the trade name of Thyrite, which has the property ofdecreasing its ohmic resistance twelve times for every doubling of thevoltage. For instance, at a potential of volts its ohmic resistance maybe 50,000 ohms while at 10,000 volts the resistance is only one-halfohm. The operating potential of 229 volts across the magnet is not greatenough under ordinary circumstances to cause the valve P and coil L topass suihcient current to actuate the relay, but when the main circuitis broken the potential across the magnet raises and since the ohmicresistance of the valve P decreases proportionately to the voltage raiseit permits the surge current to be short circuited through the coil L,energizing the same.

Energizing of the coil L moves the relay armature Q, which armaturemakes contact with the contacts R and S. Contact R is connected to thepositive side of the main line. The armature therefore controls currentflow from the main line through a bucking coil 0 and resistance U to themagnet. The other side of the magnet is connected to the negative sideof the main line through the resistance T. Hence we see that a reversecurrent at a lower potential is supplied to the magnet. The polarity ofthe magnet is changed as soon as the impedance is overcome. The contactS connects the positive side of the main line to the negative side ofthe main line through the holding coil N. The holding coil N maintainsthe relay closed until overcome by the bucking coil. A resistance V maybe provided in series with the holding coil N, the purpose being topermit a smaller and more compact winding for that coil. It is howeverquite apparent that this is merely for economy in manufacture, and theresistance could be eliminated if the resistance of the coil itself wereincreased.

Means is provided to cut oil the reversing current when the polarity ofthe magnet is reversed through the medium of the coil 0 which, is ofopposite polarity to the coil L and N. Being of opposite polarity, assoon as the current in the magnet builds up to a certain value, the coil0 neutralizes the effect of the holding coil N and permits the spring toreturn the armature Q to its original position, opening the contacts Rand S, and de-energizing the holding coil and magnet. A blow-out coil Jis provided adjacent the contact R.

If it should be desirable to provide a manually controlled reversecurrent for the magnet, this can be done by another tap on the masterswitch connected into the resistance V, by a connector W which holds thereverse contactor closed. This additional control is however optional,not being otherwise necessary to the operation of the circuit.

Having thus described my invention and embodiment thereof, I am awarethat numerous and extensive departures may be made therefrom withoutdeparting from the spirit of the invention.

I claim:

1. In a control circuit of the class described, an electro-magnet, acircuit breaker interposed between a current source and the magnet forcontrolling current flow to the magnet, said magnet being adapted to beenergized upon application of current thereto, means to eliminateresidual magnetism in the magnet upon breaking of the main circuit,comprising a subsidiary circuit for temporarily reversing the polarityof the magnet, a shunt circuit coupled to the magnet for controlling thesubsidiary circuit, and a resistance material connected in the shuntcircuit operative upon increased potential across the magnet terminalsto increase the current flow in the shunt circuit.

2. In a control circuit for an electro-magnet, a magnet, a circuitbreaker for controlling cur" rent flow to the magnet from a currentsource, an auxiliary circuit for reversing the current flow to themagnet to depolarize the same, a relay for controlling the auxiliarycircuit, means for controlling the relay comprising a shunt circuitcoupled to the magnet, and a resistance element in terposed in saidcircuit and operable upon increased voltage in the magnet to permitincreased current to flow into said relay for energizing the relay andcoupling the auxiliary circuit to the magnet.

3. In a controller for electro-magnets, a magnet, a main switch operableto connect a source of current to the magnet, a subsidiary circuitcoupledto the source of direct current for reversing current flow in themagnet, a relay for connecting the subsidiary circuit and magnettogether and having a field coil, and a shunt circuit including avariable resistance in series with the field coil and the magnet, saidresistance having the property of automatically decreasing itsresistance upon an increase in voltage to pass an increase in voltage,due to breaking the main circuit into the field coil of the relay toenergize the relay and connect the subsidiary circuit to the magnet.

4. In a control circuit for an electro-magnet, a magnet, a main circuitfor connecting the magnet to a source of current supply, a circuitbreaker in the main circuit for making and breaking the circuit to themagnet, a subsidiary circuit for reversing current flow through, saidmagnet, a circuit breaker for said subsidiary circuit to make and breakthe circuit to the magnet, and a shunt circuit connected to the magnetoperable to control the circuit breaker of the subsidiary circuitincluding an inductance adapted to actuate the circuit breaker whenenergized and a resistance in series with the inductance operable toresist current flow at normal voltage but permit current flow atabnormal voltages.

5. In a control circuit for an electro-magnet, an electro-magnet, a maincircuit for connecting said magnet to a source of current, a circuitbreaker interposed in the maincircuit for making and breaking thecircuit to the magnet, a subsidiary circuit for reversing current flowin said magnet to demagnetize said magnet, a circuit breaker interposedin said subsidiary circuit for making and breaking the circuit to saidmagnet, a shunt circuit connected to the magnet and comprising aninductance for controlling the circuit breaker in the subsidiary circuitand a resistance in series with said inductance, said resistance havingthe property of decreasing its ohmic resistance as the potential acrossit increases.

6. In a control circuit of the class described, an electro-magnet havingan inductive winding, a main circuit for connecting said magnet to asource of current, a circuit breaker disposed in the main circuit andinterposed between the magnet and the source of current, a subsidiarycircuit for reversing the polarity of the magnet to demagnetize the sameincluding a second circuit breaker adapted to connect said magnet to thecurrent source and disconnect said magnet from the current source, meansto control the second circuit breaker comprising an inductance shuntedacross said magnet and a resistance in series with said inductance, saidresistance having the property of decreasing its ohmic resistance as thepotential across it increases, a holding inductance of the same polarityas the shunt inductance and adapted to be connected into the maincircuit to hold the reversing circuit closed after the shunt circuit hasoperated, and a bucking inductance of opposite polarity in series withthe source of current and the magnet and adapted to oppose the holdinginductance when the current flowing in the subsidiary circuit builds upto a predetermined value to cause the subsidiary circuit to be opened.

7. In a control circuit of the class described, an electro-magnet havingan inductive winding, a main circuit for connecting said magnet to asource of current, a circuit breaker disposed in the main circuit andinterposed between the magnet and the source of current, a subsidiarycircuit for reversing the polarity of the magnet to demagnetize the sameincluding a second circuit breaker adapted to connect said magnet to thecurrent source and disconnect said magnet from the current source, meansto control the second circuit breaker comprising an inductance shuntedacross said magnet and a resistance in series with said inductance, saidresistance having the property of decreasing its ohmic resistance as thepotential across it increases, a holding inductance of the same polarityas the shunt inductance and adapted to be connected into the maincircuit to hold the reversing circuit closed after the shunt circuit hasoperated, and a bucking inductance of opposite polarity in series withthe source of current and the magnet and adapted to oppose the holdinginductance when the current flowing in the subsidiary circuit builds upto a predetermined value to cause the subsidiary circuit to be opened, amanually controlled switch electrically connected to the circuit breakerfor operating the same.

8. In a control circuit for an electro-magnet including main andauxiliary circuits connected to the magnet, a shunt circuit operativelyconnected with the auxiliary circuit for controlling the same, aresistance means unresponsive to low voltage but responsive to increasedvoltage connected in shunt to the main circuit and operatively connectedwith the shunt circuit for controlling the same.

9. In a control circuit for an electro-magnet, a main circuit and a maincircuit breaker in the main circuit for controlling current flow to themagnet from a current source, a reversing circuit for reversing thecurrent fioW from said source to the magnet to depolarize the same,means to close the reversing circuit connected in shunt with said magnetincluding an auxiliary circuit having a resistance operative uponincreased potential across the magnet terminals to increase the currentflow in the shunt circuit, means including a. circuit to hold saidreversing circuit closed, and means including a circuit operated inopposition to the holding circuit by a predetermined increase inreversing current through said magnet to open said reversing circuit.

10. In a control circuit for an electro-magnet, a main circuit breakerfor controlling current flow to the magnet from a current source, a.reversing circuit for reversing the current flow from said source to themagnet to depolarize the same, a relay for controlling current flow inthe reversing circuit, means to dispose of the inductive voltage in saidmagnet upon breaking of the main circuit and to initiate the operationof the reversing circuit including an auxiliary circuit having aresistance and said relay connected in series with each other andconnected in shunt with said magnet, said resistance being operativeupon increased potential across the magnet terminals to increase thecurrent flow in the shunt circuit, and circuit means in said relay tohold said reversing circuit in electrical connection with said magnetafter the relay contacts close, and a second circuit means in said relayconnected to the magnet and operable by a predetermined increase inreversing current through said magnet to oppose said holding means torelease said relay and open said reversing circuit.

ROY O. WERTZ.

